Electric furnace muffle construction



May 2l, 1935. T. H. FORDE ELECTRIC FURNACE MUFFLE CONSTRUCTION Filed Ooi. 30, 1951 9 4 a ff f A 2 oje,

NH2/LE?. 3.

INVENTOR. 7770/7705 fZ/'de 0' ATTORNEY.

Patented May 21, 1 935 i UNITED STATES PATENT OFFICE A ELECTRIC FURNACE MUFFLE CONSTRUCTION Thomas H. Forde, San Francisco, Calif., assgnor to Diascope Corporation Ltd., San Francisco, Calif., a corporation of California Application October 30, 1931, Serial No. 572,110

1 Claim. (Cl. 219-36) The invention relates to improvements in elecand preferably made of an asbestos cement comtric furnace muiiles and more particularly to the position. The inner wall l which denes the construction and arrangement of the muiile Walls. heating chamber of the muie extends completely An object of the invention is to provide a muille around the top, bottom,Y and opposite sides of the 5 wall construction in Which will be avoided the niufe and has formed on the outer side thereof 5 necessity of replacing the heat insulation material a series of circuniferentially extending grooves thereof, due to excessive muiile temperatures and 9 in which is disposed the electrical element I2 the resultant burning or damage of the material. for heating the chamber. Electrical connection Another object of the invention is to provide a with said element is afforded from the rear por- 10 muflle wall construction with the above feature, tion of the munie through a pair of conductors l0 which may be made at practically no more ex- I3. At the front of the muiile is provided the pense than such type of walls as heretofore conopening it. which affords access to the chamber Strueted, and will in addition afford greater and is arranged to be closed by a suitable door I6. economy in eiiciency of operation as well as in Heretoore, in the design 0f muieS, the entire l5 maintenance cost. space between the inner and outer walls was filled 15 A further object of my invention is to provide with a heat insulating material usually 0f a a muiile wall construction which will act to posifibrous nature, and as a result, after the muille tively space the insulation material from the Was ill SelViCe 1 comparatively SnOlt time and heating coil winding and the muilie chamber particularly When eXCeSSiVe temperatures Dre- Walls, and thus prevent disintegration of the same vailed in the heating element, the insulation m2- 20 caused by flexing or amalgamation of the insulattel'ial, flTSt the DOltiOnS at the inner Weil and ing material with said walls and windings, then the portions more remote, slowly chemical- A stiii further object of my invention is to pro- 1y united with the windings of the heating ele- Vide a inutile wall construction Surrounding the ments and the refractory material on which the heating unit and so arranged as to reeot heat element was wound to subsequently render the 25 radiations emanating from said unit back into element and SllDlJOlt tnel'efOl Useless, whereby the central portion of the mtime a dismantling of the entire muffle and a replace- The invention possesses other objects and feament Of the innel Wall and the Winding thereon tures of advantage, some of which, with the forebecame necessary With the Intime Well construe* going, will be set forth in the following descriptien 0f my invention, Such damage t0 the heat- 30 tion of the preferred form of the invention which ine unit, that S the inner Walls and windings, and is illustrated in the drawing accompanying and the necessity of replacement are entirely avoided. forming part oi the speoinoation. it is to be As will be Clear from Figure 2, there is interunderstood, however, that variations in the showposed between the inner and Outer Wells en ining made by the said drawing and description termediate wall Il which is preferably disposed 35 may be adopted within the scope of the invention in close proximity t0 but in spaced relation from as set forth in the claim. such wall so as to define therebetween a com- Referring to said drawing: paratively narrow air space or chamber I8. The

Figure l is a vertical sectional view of a mufile Wall IT iS preferably OImed 0f glazed leflaCtOry having the walls thereof constructed in accord, material and is in the nature of a shield for the 40 ance with my invention. insulating material I9 which fills the compara- Figui-e 2 is an enlarged horizontal section of tively Ierse Space 2i between Such Wall and the the mufle taken on the line 2-2 of Figure i. outer wall 8. As the chamber I8 and Wall Il Figure 3 is an enlarged fragmentary vertical are aS here ShOWIl COTnDelatiVeiy DeilOW, PlaCti- Section of the munie taken at right angles to cally the full space between the inner and outer 45 Figure l. Walls is still available for the insulating mate- AS illustrated in the drawing, the mule 4 is in rial and in addition the trapped air in chamber the present form of the invention arranged to i8 Of itself provides a heat insulating medium 0f removably and operatively iit upon a base 6 of an extremely low conductivity. It will now be clear electric furnace unit particularly designed for that the interposition of the wall Il between the use in the dental profession and in dental laborinsulating materiel and the heating COl Windatories. The muffle is formed as most clearly ings effectively prevents these materials from shown in Figure 2 with an inner wall 'l prefercontacting and therefore eliminates the harmful ably of a high heat resistance refractory matechemical union hereinbefore referred to. There 5 rial, and an outer Wall 8 spaced from the wall l is, however, an additional function of the wall I1 55 25 Y manneris comparatively small; As the temper-2 atr'e'jofithe unitis increased thelatter Will begin" which comprises the reflection of heat'radiations emanating from the heating element when the latter and the refractory on which it is Wound are heated to such a degree that the frequency of certain of the heat radiations therefrom extend Well into the visible spectrum. It AWill of course be clear that as the heating unit warms, the heat therefrom may escape outward only by the three-methods of heat transfer to- Wit, con-`r vection, conduction and radiation. Since theai'r' surrounding the heating unit is trapped, and the air space itself too narrow to permit appreciajf ble circulation, the transfer of heat byconvection from the heating unit to the Wall I-'l lis extremely slight. It Will further be noted that the outward transfer of heat through the airspacebycon` duction is also small due Atoritheiverylowthernial"A conductivity of air. Heat radiations, fhovvever';V emanating from the heating unit undulate` through the ether and are therefore freely`tr'ans mitted through the air space surroundingfthe heating unit. At low temperatures the frequency of j the f heat 'radiations' are comparatively v"loW;

andsinceradiation'is a functioniof both the tem; l

perature andthe frequency," the heat lost' in this tcfglo'w until at"'a maximumtemperature,` the heatin'g'iunit emanates a considerable amount 'of light.' ','Under this 'latten'condition"thereis an f, extremely large heat radiationfrom theunit."

u Hetofor'e" 'in theconstruction' of mullles the inslllatinsurrounding the heating unit readilyk absorbed the heat radiation 'froinfthe heatingv unit and thus this heat was entirely'lost. In'my" 3 muifle,'however, the Walll'l l yis arranged to reflect "tweedaagse bak wpihej heating. #mii- In order that the wall may so function it ispreferably made of a white plastic compound With a smooth glazed inner surface. Inasmuch as the air space acts to prevent the transfer of heat by either convection or conduction to the inner side of this wall the same Will remain comparatively cool and maintain its smooth, light colored refleeting surface. On the other hand since the air spaceV freelygtransmits radiatedrheatv, thereV is practically rioneofg this ietfabsorbedinthe air space and thus practically all of the radiated heat reflected from the inner surface of the Wall l] is confined .and convertible into useful energy.

jlji'eferablythe intermediate Wall or shield Il, asvtellas the inner Wall, extends into and re- Vmovablynts'ina groove 2l in the front wall porcasingjv and'lan intermediate casing of 1`olast'i(":"V compositioniminediately surrounding'said'm" y, casing and spacing therefririsaidinsulation ima A tener" the? inner sidej or said intermediate easing@ facingf'said muiile -casingbeing spaced therefro and glazed tofrevlectlbackJtosaid u'lec radiantlleat from saidmuflle'caisin Mfg Y 'rHQMAe Formia.f 

